The demand for wireless communications services has increased significantly, both for voice and data services. To meet the increased demands, new wireless technologies have been developed. For example, in the third generation partnership project (3GPP) wideband code division multiple access (WCDMA), high speed downlink packet access (HSDPA) and high speed uplink packet access (HSUPA) in Release 5 and 6, respectively, have been introduced to achieve a significant increase in spectrum efficiency and peak data rates.
The wireless signal propagated over the air is subject to various signal impairments, including propagation losses, shadowing, multipath fading, Doppler shifts, etc. The multipath fading or fast fading is caused by the combination of the replicas of the transmitted signal with varying phase and amplitude due to reflections on objects encountered in the propagation paths. Multipath fading results in undesirable fluctuations of the received signal power.
A transmit diversity scheme has been developed to cope with the negative effects of fading. A transmit diversity is a scheme of transmitting the same signal over multiple independent paths. The transmit diversity is implemented by sending the same signal at different instants in time (time diversity), over different frequency carriers or subcarriers (frequency diversity), or over different antennas (space diversity). Downlink transmit diversity, both closed loop and open loop, are part of the WCDMA specifications.
Multiple antenna techniques, such as transmit diversity/beamforming or multiple-input multiple-output (MIMO), have not been adopted into the HSUPA. Enhanced uplink performance is important for reducing the WTRU transmission power requirements, especially for high data rate applications. In addition to reduced WTRU battery consumption, an improved UL performance translates into better coverage area for the high data rate services.
A power control is an important factor for interference management in the interference-limited multiuser communication systems, particularly for code division multiple access (CDMA)-based HSUPA system. In such systems, performance of each user depends not only on its own transmission, but also on the transmissions of other users. Conventional power control mechanisms for HSUPA and WCDMA uplink are based on single-input single-output (SISO) system, where only one antenna is used at both the transmitter and receiver ends.